Referring to FIG. 1, and in accordance with some well cementation procedures, a well (e.g., an oil well) 100 is drilled into a formation 107 from a surface (sea floor if an offshore well) 103 to a depth designated as being well bottom 104. Once drilled, the drill string is typically removed, thereby leaving the well filled with drilling fluid. A casing string 101 is then run down well (or hole) 100, thereby establishing annular region 112 between the outer casing wall and formation 107. Typically, a guide shoe 109 and centralizer (not shown) are attached to the bottom of casing string 101. Guide shoe 109 generally comprises an orifice 115 from which fluids flowing down the interior of casing string 101 can emanate and flow back up through annular region 112. Additionally, a float collar 111 can be positioned in casing string 101 near the well bottom to inhibit fluid flow back up the interior of the casing string.
Still referring to FIG. 1, drilling fluid present when the well is cased can be displaced using a drilling fluid 106 (or other suitable displacement fluid) passed down casing string 101, through float collar 111 and guide shoe orifice 115, and up annular region 112. Because cement slurries are typically incompatible with drilling fluids, the drilling fluid 106 can be followed by a spacer fluid 108 during a cement job. Note that turbulent “spacer(s)” are sometimes pumped with laminar spacer(s), such spacer combinations often being referred to as “spacer trains.”
With continued reference to FIG. 1, following the spacer fluid, a cement slurry 110 (sometimes pumped with a “lead slurry” and a “tail slurry”) is introduced down the casing string between a penetrable/rupturable bottom plug 102 and a solid top plug 105. Following the top plug 105, a displacement fluid 118 (e.g., drilling fluid), sometimes being preceded by a spacer fluid, forces the bottom plug/cement slurry/top plug system down casing string 101 until the bottom plug reaches float collar 111, at which point bottom plug 102 ruptures and cement slurry 110 flows through float collar 111, out orifice 115, and up annular region 112 until top plug 105 lands on bottom plug 102 (signaled by a rise in pressure). Note that those of skill in the art will appreciate that a high degree of variability exists in the above description of well cementation (e.g., multiple bottom plugs, graduated fluid densities, etc.), and that the description above should not be seen as limiting the scope of the invention described hereinafter (vide infra).
Further regarding the incompatibility issues mentioned above, cement slurries are, more often than not, chemically incompatible with drilling fluids. In fact, the mixing of such incompatible fluids can result in a highly-viscous, highly-gelled mixture that can be difficult, if not impossible, to displace from a well. To prevent intermixing of incompatible combinations of drilling fluids and cement slurries in a well, spacer fluids have long been used to mitigate against such intermixing. See, e.g., Weigand et al., U.S. Pat. No. 4,588,032; and Wilson et al., U.S. Pat. No. 5,113,943.
The primary requirement for cement spacer fluids is that they be compatible with both the drilling fluid and the cement slurry that they are used in conjunction with. Additionally, the spacer fluids should possess certain rheological tendencies, such as turbulent flow at lower shear rates, which assist in granular solids removal and which encourage the removal of the drilling fluid filter cake from the walls of the well. Turbulent flow is generally regarded as the most effective method for well cleaning during cementing operations.
Conventional cement spacers are typically composed of an aqueous base fluid and a densification agent. Other components can include, for example, one or more of the following: anti-settling agents, dispersal agents, fluid loss controlling agents, viscosifying agents, and the like. For aqueous-based spacer fluids, all of such one or more additional components should be soluble and/or dispersible in water. Furthermore, in some instances a single component additive may impart a plurality of properties to the resulting fluid mixture.
The density (or weight) of a cement spacer fluid should be variable and will typically be adjusted according to well control and compatibility parameters associated with the particular drilling fluid and cement slurry with which it is associated. In some instances, where there is a density mismatch between the drilling fluid and the cement slurry, the spacer fluid is densified such that it is intermediate between that of the drilling fluid and the cement slurry. Additionally, the density of the spacer fluid can be graduated to better match the densities of the fluids between which it is interposed. See, e.g., Wilson, U.S. Pat. No. 5,027,900. For turbulent flow, the density of the cement spacer fluid is typically limited to ˜10 pounds per gallon (ppg) using traditional densification methodologies (e.g., saturated NaCl brine).
Cement slurries are typically more viscous than the drilling fluids preceding them in a given cement job, and spacer fluids have historically had viscosities that are typically intermediate to that of the drilling fluid and cement slurry they are used in conjunction with, wherein the relatively high viscosity of such spacers generally requires that they be pumped under laminar flow. The viscosity of the cement slurry is also largely a function of the various components added thereto. To retain the desired rheological properties and permit turbulent cleaning of the well, such spacer fluids should generally have a relatively low viscosity (e.g., ˜5 centipoise (cP) or less).
Historically, cement spacers have been densified by adding viscosifying agents and/or non-soluble weighting agents to fresh water, seawater, brines, or other aqueous or non-aqueous base spacer fluids (higher viscosity is needed to support the dispersion of the weighting agents). The resulting fluids, however, are either high viscosity Newtonian fluids, Bingham plastic fluids, power law fluids, or modified Hershel-Bulkely fluids—all of which are incapable of being placed in turbulent flow, at achievable rates, around the entire annular region. Accordingly, such fluids must be pumped in laminar flow to maintain well control and effective cementation of the annular space.
In view of the foregoing, a method or system for easily densifying a cement spacer fluid (e.g., to ˜10 ppg or more), while keeping the fluid Newtonian and pumpable under turbulence, would be extremely useful.